This paper presents a new multi-objective control strategy for inverter-interfaced distributed generation (IIDG) to ensure its safe and continuous operation under unbalanced voltage sags. The proposed control strategy can effectively improve the low voltage ride through (LVRT) capability, reduce active power oscillations, and limit overcurrent simultaneously, which are marked as the most important control objectives of IIDG during unbalanced voltage sags. The advanced voltage support scheme, which utilizes positive sequence component, is firstly proposed to maximize the LVRT capability of IIDG during unbalanced voltage sags. Then, to ensure the safety of IIDG, the active power oscillation suppression and current limitation algorithm are designed individually. Based on the control algorithms of such objectives, the multi-objective control method, including scenario classification and reference current determination, is then presented to achieve such three objectives under various system conditions simultaneously. Finally, case studies and evaluations based on MATLAB/Simulink are carried out to illustrate the effectiveness of the proposed method.

Standard back propagation (BP) neural network has disadvantages such as slow convergence speed, local minimum and difficulty in definition of network structure. In this paper, an learning vector quantization (LVQ) neural network classifier is established, then it is applied in pattern classification of two-dimensional vectors on a plane. To test its classification ability, the classification results of LVQ neural network and BP neural network are compared with each other. The simulation result shows that compared with classification method based on BP neural network, the one based on LVQ neural network has a shorter learning time. Besides, its requirements for learning samples and the number of competing layers are also lower. Therefore it is an effective classification method which is powerful in classification of two-dimensional vectors on a plane.

Overvoltage may occur in low-voltage direct current (LVDC) distribution system during fault recovery, and cause permanent damages to the vulnerable power electronics devices. To suppress the overvoltage, the parameter matching principle of the inductance type superconducting fault current limiter (SFCL) is proposed. First, by deriving the equivalent circuit of LVDC system during fault recovery, mechanism and related factors of the overvoltage were studied. It is found the inconsistent drop rate of voltages of converters during the fault is the main reason for fault recovery overvoltage. Then, the method was proposed by designing the inductance in accordance with the capacitance of each converter with keeping voltage drop rate of the converter is the same and the fault current is within the breaking ability of DC circuit breaker. Finally, the method is validated by simulation in PSCAD/EMTDC, resulting in the overvoltage is reduced by 50%.